This invention generally relates to variable light transmitting systems and more particularly to those devices used in applications where it is desired to electrically select a low transmission state and then maintain this transmission state for prolonged periods. The invention is especially useful with mirror systems utilizing continuously variable reflectance elements, such as electrochromic elements. The invention also finds application in vehicle sunroofs and windows and in other glazing found in buildings and offices such as privacy and security panels in office partitions, vehicle windows and the like.
Variable transmission devices, such as electrochromic windows and mirrors, color under the application of electrical command signals. Such devices exhibit a continuously variable transmission characteristic whereby the transmission in the dimmed, or colored, state is determined by the construction and design of the device, and by the level of signal applied thereto. In such devices, a partial light transmission level is selectable from a continuum of light transmission levels in a range from a highest light transmission level to a lowest light transmission level. Electrochromic variable transmission devices include in their construction materials that color under an applied electrical voltage drive or current. Many device constructions are possible, as disclosed in "Electrochromic Automotive Day/Night Mirrors" SAE Publication 870636, published February 1987, by Niall R. Lynam, and "Smart Windows for Automobiles" SAE Publication 900419, published February 1990, by Niall R. Lynam. Such variable transmission mirrors and windows have a plurality of possible applications. Several of the applications require that the device be colored to a desired lower partial transmission state (or reflectance level in the case of mirror devices) and that this desired lower transmission state be maintained for prolonged periods ranging from several minutes in some applications to several hours or days in others. For example, where the application is a variable transmission window such as an electrochromic sunroof, it would be desirable to dim the transmission of solar radiation through such window by coloring the variable transmission device to some desired low transmission level and then maintain the selected low transmission level for several hours, even while the vehicle is parked or when the vehicle is operated during a journey of extended distance
In the case of a rearview mirror in a vehicle, it would be desirable at night to dim the reflectance level of the mirror to a selected lower reflectance level to provide protection from glare produced by following headlamps. In another example, where the variable transmission element is an architectural window such as is found in office buildings, homes, and the like, it would be desirable to select some low transmission level during periods of prolonged intense solar radiation. In the examples above, it is contemplated that, once a desired transmission state is selected, and once this desired lower transmission state is achieved by applying an appropriate voltage to the variable transmission device, this selected lower transmission state is maintained (or as close to the selected lower transmission state as is consumer discernible) for a prolonged period of at least some minimum period such as several minutes, and often for several hours.
The level of transmission to which an electrochromic device dims is a function of several factors, including the design and construction of the device as well as the applied voltage signal level. Any level of light transmission within the achievable range of transmission levels for the particular device can be accessed by applying the appropriate drive signal level. Upon application of that signal, electrochromic reactions are stimulated within the electrochromic materials, reactions that typically involve redox reactions and some type of ion or molecular transport. Conventionally, the drive signal appropriate to stimulate a determined level of electrochromic reaction, and thereby achieve the selected desired lower transmission state, is maintained throughout the period over which the particular lower transmission state is desired. Such electrochromic devices are typically returned to a higher transmission state, typically known as bleaching, by applying a signal of reverse polarity to that used to color the device to the dimmed, lower transmission state. Alternatively, the electrochromic device may be bleached by applying 0.0 V across the device, such as by short-circuiting its electrodes.
The electrochromic reaction that leads to the coloration of the transmission device may not be the only electrochemical reaction occurring within the device. Side reactions are possible, with the probability of occurrence increasing with the level of applied drive signal required to increase the coloration of the device. The side reactions often lead to device degradation and noticeable cosmetic defects. Therefore, when the drive signal is applied to the device for prolonged periods, the device reaches a steady state level of light transmission, beyond which there is no additional beneficial electrochromic coloring effect. However, the continued application of the drive signal continually drives the side-reactions so that the long-term reliability of the device may be impaired. Additionally, coloration of the electrochromic device often includes dual injection (or ejection) of electrons and ions. Beyond a certain level of electron and ion injection/ ejection, any further injection/ejection can lead to a diminishing incremental coloring efficiency and lead to problems of reversing the reactions when it is desired to bleach the colored layers.
Also, electrochromic devices typically dim when a DC potential is applied; the voltage level is typically in the 0.5 V to 2.0 V range. However, the voltage source available in automobiles is typically 12 V DC and that available in buildings is usually 110 V AC (220 V AC in Europe). Thus, voltage reduction means and, in the case where the power source is AC, rectification and smoothing means, must be utilized to convert the voltage source available to the DC lower voltages needed to power EC devices. Thus, any current leakage whatsoever during prolonged coloration while voltage is applied due to electrochemical side reactions, pinholing that leads to micro-shorts, leakage currents across dielectrics, and the like is particular energy inefficient in that, besides not contributing to a consumer appreciable electrochromic effect, power is wasted in the power supply circuitry utilized to provide said electrochromic coloring voltage. Therefore, continued application of a drive signal in order to achieve a particular level of light transmission is considered detrimental.
Many electrochromic devices retain their coloration state even when the drive signal, which has been applied to achieve the coloration state, is removed. Electrochromic devices that have this property, commonly known as memory, are energy-efficient. Electrochromic devices that have good memory typically involve a thin-film (usually an inorganic metal oxide such as tungsten oxide or nickel oxide, or an organic thin film such as Prussian Blue or polyaniline) that colors electrochromically when a voltage is applied thereto, separated from a counterelectrode (which itself may be electrochromic) by an ion-conducting, electron-insulating electrolyte. This electrochromic combination is, in turn, typically sandwiched between electron conducting electrodes, one at least of which is usually a transparent conductor such as indium tin oxide.
When a voltage is applied to the device, the electrochromic layer usually colors by dual injection/ ejection of electrons and ions. As such, the electrochromic layer acts as a charge storage layer making the electrochromic device function in a manner analogous to a capacitor. For a given applied level of voltage, a given charge builds up within the electrochromic layer and, commensurate with the amount of charge built-up, the electrochromic layer colors. Upon removal of the applied, charging voltage such that the device electrically "floats", the charge remains within the electrochromic layer and the electrochromic device remains colored. The duration over which the charge that is built up in the electrochromic layer remains after removal of the charging potential is finite. Eventually, the stored charge leaks away and the light transmission level steadily increases until the electrochromic device returns to a fully bleached state. The period over which this occurs can vary between several minutes for some electrochromic devices to several hours or even days for others.
Therefore, as described above, when an electrochromic device has appreciable memory, it is unnecessary and undesirable to continually maintain the coloring voltage applied once the device has colored to its targeted lower transmission state. Thus, if an electrochromic device had infinite memory, or, alternatively, if the memory was so long that no appreciable upward drift in transmission level was detectable by the consumer or discernible over the time period over which the targeted lower transmission state was desired maintained, then it would suffice to simply color the device to the lower transmission level, and once this was reached, then remove the coloring voltage so that the electrochromic device floats. Thereafter, memory would maintain the desired lower transmission level until such time as a change to some other transmission level was desired whereupon an appropriate bleaching (or coloring) potential would be reapplied. However, not all devices have sufficiently long memory for the device memory itself to be relied upon to sustain a selected lower transmission level indefinitely. Charge leaks from the colored electrochromic medium by several routes such that bleaching, at a very slow rate for long memory devices and at a somewhat faster rate for shorter memory devices, commences immediately the coloring voltage is removed.
U.S. Pat. No. 4,298,870 issued to Saegusa discloses a technique for utilizing the memory property in an electrochromic display element to intermitently drive the display element. Means are provided for detecting the quantity of charge stored in the display element and for generating a detection signal when the detected value of the stored charge is below a predetermined value. Drive means are provided to apply a voltage signal to the display element when a display command signal and the detection signal are both present. In this manner, the drive signal is applied intermittently to the display element. While the principles set forth in the Saegusa patent work well for a display device in which the electrochromic layer is either colored or not colored, it does not provide a mechanism for utilizing the memory property of electrochromic elements with either a window device or a rearview mirror in a vehicle, wherein the degree of coloration of the light transmission element is continuously variable within a range of values.
Thus, a need clearly exists for means to allow selection of some dimmed state in transmission of an electrochromic device and to ensure that the transmission of the device is maintained, over a prolonged time period, close to that selected dimmed transmission state. Further these means should greatly reduce the severity of the energy inefficiency, degradative side-reaction; and allied disadvantages that accompany prolonged application of a coloring voltage to maintain said selected dimmed transmission level. Also, these means should be economical to manufacture and utilize and should be especially well-suited to the cost and performance expectations normal for automobile components. Finally, these means should perform satisfactorily regardless of the operating conditions, including its temperature of operation, of the electrochromic device, and these means should operate satisfactorily over the entire device lifetime.